The largest crystals in the world at Cueva de los Cristales aka Cave of Crystals, one of the most extreme places on the planet discovered in 2000 by two brothers drilling for silver at Naica Mine, in Chihuahuan Desert. Mexico.
Air Temperature of 50C(122F) + Relative Humidity of over 90% = Humidex Value of 105C (228F)
The giant crystals of the main chamber are made mainly of Gypsum and under extremely rare conditions were allowed to grow unimpeded
Cueva de los Cristales lies above an intrusion of magma about a mile below the surface. For about 600,000 years mineral-rich water filtered through the cave, depositing molecules of calcium sulfate like stacked bricks. At first an intrusion of magma deep below the surface superheated the water. Then around half a million years ago the water temperature cooled to roughly 136 degrees Fahrenheit, the right temperature for the calcium sulfate in the water to form selenite crystals. Conditions in the water-filled cave remained virtually unchanged for hundreds of thousands of years, allowing the crystals to reach astounding sizes.
Around 1985 Naica’s miners -owned today by the Peñoles company- unknowingly drained the water from the cave as they lowered the water table in the mine with pumps. No longer immersed, the crystals stopped growing.
Carsten Peter @ National Geographic